1. Field of the Invention
The present invention relates to the protection of integrated circuits against electrostatic discharges.
2. Discussion of the Related Art
An integrated circuit comprises metal pads intended to provide connections to the outside. Some of the pads are capable of receiving power supply voltages. The other pads are capable of receiving and/or of providing input-output signals. Power supply rails, connected to the supply pads, are generally provided all around the circuit to power its different components. Generally, an insulating layer covers the circuit, only leaving access to the metal pads.
Such a circuit generally receives and/or provides signals of low voltage level (for example, from 1 to 5 V) and of low current intensity (for example, from 1 μA to 10 mA), and is capable of being damaged when overvoltages or overcurrents occur between circuit pads.
It is thus provided to associate a protection structure with each pad. The protection structure should be able to rapidly carry off significant currents, likely to appear when an electrostatic discharge occurs between two pads of the circuit.
FIGS. 1, 2A, and 3A show three input/output pads IO1 to IO3 and a high power supply pad VDD of an integrated circuit. Each pad is coupled with a protection structure connecting the pad to a ground terminal (GND) of the circuit. “Ground” designates, here and in the following description, a reference voltage common to various components of the integrated circuit, for example, a low power supply voltage. The ground connections can be performed via a ground rail, or low power supply rail, this rail being connected to a pad, accessible outside of the circuit and capable of being set to the selected reference voltage.
In FIG. 1, each pad is coupled with a protection structure 1 comprising an NPN-type bipolar transistor 5 having its collector C and its emitter E respectively connected to the pad and to ground, and having its base B grounded via a resistor R0.
To enable to drain off the current between two pads of the circuit, each protection structure 1 associated with a pad should be able to remove overvoltages between the pad and the ground, whatever the biasing (positive or negative) of the overvoltage.
In case of a negative overvoltage between a pad and the ground, the base-collector PN junction of transistor 5 associated with the concerned pad, forward biased, becomes conductive, and the overvoltage is removed.
In case of a positive overvoltage between the pad and the ground, the base-collector junction of transistor 5, reverse biased, becomes conductive by avalanche effect, and the current flows towards the ground, through resistor R0. As soon as the voltage across resistor R0 reaches a given threshold, the base-emitter PN junction, forward biased, becomes conductive, and the overvoltage is removed.
A disadvantage of protection structures with bipolar transistors is that, in the case of a positive overvoltage between a pad and the ground, they have a poorly controlled trigger voltage (breakover voltage of the base-collector junction). Thus, there is a risk for components of the integrated circuit to be destroyed before the protection starts operating. Further, to withstand the power dissipated in an electrostatic discharge, bipolar transistors 5 should have large dimensions, which poses problems in terms of silicon surface area and stray capacitances.
FIG. 2A shows a protection structure with Shockley diodes. Each pad is coupled with a Shockley diode 11 forward-connected between the pad and the ground.
FIG. 2B shows an equivalent electric diagram of a Shockley diode, or thyristor with no gate terminal. In this thyristor, the cathode gate region is connected to the cathode. To enable the reverse conduction of the structure, a resistor Rs is provided between the anode gate region and the thyristor anode.
In case of a negative overvoltage between a pad and the ground, the PN junction between the cathode gate region and the anode gate region, forward biased, becomes conductive and the overvoltage is removed through resistor Rs.
In case of a positive overvoltage between the pad and the ground, the thyristor becomes conductive by breakover of the PN junction formed between its cathode gate and anode gate regions, and the overvoltage is removed.
An advantage of protection structures with thyristors is that they have much greater overvoltage removal performances than structures with bipolar transistors. This is especially due to the very low voltage remaining across the thyristor, when the latter is made forward conductive. For equivalent overvoltage removal possibilities, the dimensions of thyristor protection structure 11 of FIGS. 2A and 2B are much smaller than the dimensions of bipolar transistor protection structure 1 of FIG. 1. As a result, protection structures with a thyristor generate much smaller stray capacitances than bipolar transistor protection structures.
However, in the same way as for protection structures with bipolar transistors, a disadvantage of protection structures with thyristors of the type described in relation with FIGS. 2A and 2B is that they have a poorly controlled trigger voltage (breakover voltage of the Shockley diode). Further, since the semiconductor areas forming the diodes are formed of doped regions provided to optimize the active components of the integrated circuit, it is difficult to obtain optimal breakover voltages and too high break over voltages result being obtained. Thus, there is a risk for components of the integrated circuit to be destroyed before the protection starts operating.
FIG. 3A shows another example of a structure of protection against electrostatic discharges. In this structure, diodes 21 are forward-connected between each input-output pad (IO1, IO2, IO3) and a high power supply rail 20 connected to pad VDD. Diodes 23 are reverse-connected between each input/output pad and the ground (GND). In the vicinity of each pad, a MOS transistor 25, used as a switch, is connected between high power supply rail 20 and the ground. With each transistor 25 is associated an overvoltage detection circuit 27, connected in parallel with transistor 25, and capable of providing this transistor with a triggering signal. Each MOS transistor 25 comprises a parasitic diode 26, forward-connected between the ground and high power supply rail 20.
In normal operation, when the chip is powered, the ground voltage and the signals on the input/pads and on high power supply rail 20 are such that diodes 21 and 23 conduct no current. Further, detection circuits 27 make MOS transistors 25 non-conductive.
In case of a positive overvoltage between pad VDD and the ground, circuits 27 turn on transistors 25, which enables to remove the overvoltage.
In case of a negative overvoltage between pad VDD and the ground, parasitic diodes 26 of transistors 25 become conductive and the overvoltage is removed through these diodes.
In case of a positive overvoltage between an input/output pad and pad VDD, diode 21 associated with the concerned input/output pad becomes conductive, which enables to remove the overvoltage.
In case of a negative overvoltage between an input/output pad and pad VDD, circuits 27 turn on transistors 25, and the overvoltage is removed through transistors 25 and through diode 23 associated with the concerned input/output pad.
In case of a positive overvoltage between an input/output pad and the ground, diode 21 associated with the concerned pad becomes conductive and the positive overvoltage is transferred onto high power supply rail 20, which corresponds to the above case of a positive overvoltage between pad VDD and the ground.
In case of a negative overvoltage between an input/output pad and the ground, diode 23 associated with the concerned pad becomes conductive, which enables removing the overvoltage.
In case of a positive or negative overvoltage between two input/output pads, diodes 21 or 23 associated with the concerned pads become conductive, and the overvoltage is transferred between high power supply rail 20 and the ground, which corresponds to one of the above overvoltage cases.
FIG. 3B shows in further detail a possible embodiment of a circuit 27 for detecting a positive overvoltage between high power supply rail 20 and the ground (GND), and for controlling a MOS protection transistor 25. An edge detector, formed of a resistor 31 in series with a capacitor 33, is connected between high power supply rail 20 and the ground. Node M between resistor 31 and capacitance 33 is connected to the gate of a P-channel MOS transistor 35 having its source connected to high power supply rail 20 and having its drain grounded via a resistor 37. Node N between the drain of transistor 35 and resistor 37 is connected to the gate of protection transistor 25. An assembly 39 of diodes in series is forward-connected between node M and the ground. In this example, assembly 39 comprises four diodes in series.
In normal operation, when the circuit is powered, node M is at a high state. P-channel MOS transistor 35 is thus off. Thus, gate node N of transistor 25 is at a low state, maintaining this transistor off. When the voltage difference between high power supply rail 20 and the ground increases, the voltage at node M also increases. When the voltage at node M reaches a given threshold, diode assembly 39 becomes conductive. In this example, if each diode has a 0.6-V threshold voltage, assembly 39 turns on when the voltage at node M exceeds 2.4 V. The voltage at node M thus stops increasing, while the voltage of high power supply rail 20 keeps on increasing, which turns on P-channel MOS transistor 35. Thus, gate node N of protection transistor 25 switches to a high state, that is, substantially to the same positive voltage as pad VDD. Transistor 25 thus turns on, and the overvoltage is removed.
When the integrated circuit is not powered, node M is in a low state. Since transistor 35 is not powered, node N of this transistor is in an undetermined state. If an abrupt positive overvoltage (fast voltage rise) occurs between pad VDD and the ground, node M remains in a low state. Transistor 35 thus turns on and node N switches to a high state. Thus, protection transistor 25 is turned on, and the overvoltage is removed.
A disadvantage of the protection structure of FIGS. 3A and 3B lies in the fact that, to be able to drain off the currents induced by electrostatic discharges, diodes 21 and 23 and transistors 25 should have a large surface area (for example, a 200-μm junction perimeter per diode 21, 23 and a channel width often greater than 1,000 μm per transistor 25, for example, on the order of from 1,000 to 10,000 μm). As a result, a significant silicon surface area is exclusively dedicated to the protection against electrostatic discharges, to the detriment of the other circuit components. Further, due to their large dimensions, MOS transistors 25 have high stray capacitances, and conduct significant leakage currents in the off state.
Other protection structures comprising a diode coupled to a thyristor have been described in international patent application WO2006/033993 and in US patent US2002/0089017.